Let's imagine for a second that you have all the time in the world (most people don't) and you are free to practice as long as you want.
Here is the question: are these long practice sessions helpful in the long run? Can you really stay focused all seven hours on the organ bench and put in all your effort?
I think that it is counter-productive to practice for seven or more hours a day regularly.
I personally have practiced that much or even more when I had to prepare for a recital in 2 days. That was a marathon practice. Very difficult for the mind to endure. But I had no other choice.
However, I would say an organist can achieve good results with 2 hours of regular, focused and wise practice. It's not really the length but quality of the organ practice that matters.
After 2 hours, your mind and body gets weak and you really have to push. So I don't recommend for the majority of people.
I think it's also important to make breaks of 5-10 minutes every 30 minutes or so (drink a glass of water, stretch etc.). Then you will feel refreshed and never tired.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my video Organ Practice Guide.